This Thursday, for all inhabitants of the northern half of the planet, is the summer solstice. It’s the longest day of the year north of the equator, and it’s the start of summer.
Earth has a solstice every six months, in June and December. In spring, the sun seems higher in the sky, at the same time every day, until it reaches its zenith (this Thursday).
Starting Friday, the days will get shorter. The sun will be a little lower in the sky each day until the winter solstice on December 21.
What time is the summer solstice this year?
The summer solstice will occur this afternoon at 4:51 p.m. Eastern Daylight Time. This is the time of day when the sun rises closest to the North Celestial Pole during the year.
The height of the sun in the sky varies each day because the Earth rotates on an axis tilted 23.5 degrees from the vertical. This means that, depending on the time of year, the hemispheres lean towards the sun or away from it.
This is what gives Earth its seasons: when the northern half of the planet tilts toward the sun, it’s summer; at the same time, the southern half of the planet is moving away from the sun and it is the southern winter. Why is the Earth tilted like this? It remains a mystery. But some astronomers think this tilt made life possible.
In the Northern Hemisphere, the summer solstice occurs when the Earth’s tilt relative to the sun is greatest. However, the extreme heat generally occurs later, in July or August, while the sun’s rays warm our part of the planet.
Every tilted planet has solstices. All planets in our solar system have an axial tilt, but some are more pronounced than others.
Venus and Jupiter have a slight tilt, around 3 degrees. Mercury, at 0.03 degrees, is barely tilted. In contrast, Uranus is oriented 97.8 degrees from vertical; therefore one of its poles sometimes points directly towards the sun. This extreme tilt causes some of the most spectacular seasons in our solar system.
Both Saturn and Neptune have a tilt close to that of Earth. Just like Mars, with 25.2 degrees (the Red Planet’s tilt has varied greatly over the past few million years).
Every year, people around the world celebrate the June solstice with bonfires and festivals. Above the polar circle, we can celebrate the midnight sun. Halfway around the world, scientists living in Antarctica hold an annual mid-southern winter celebration to mark the continent’s longest and darkest night.
Others travel to ancient ruins, like Stonehenge in England or the Temple of the Sun in Peru, to greet the solstice sun as ancient people once did.
